The present invention relates to masks and projection exposure apparatuses, in particular, those suitable for a photolithography process for fabricating various devices including semiconductor devices such as ICs and LSIs, image pick-up devices such as CCDs, display devices such as liquid crystal panels, and magnetic heads.
A photolithography process for fabricating semiconductor devices uses a projection exposure apparatus to transfer a circuit pattern formed on a reticle or photo mask (called “a mask” hereinafter), onto a photosensitive material applied semiconductor wafer (or a photosensitive plate). Importantly, the projection exposure apparatus needs to use such a projection optical system as exhibits excellent image-forming performance with small aberration, so as to transfer the mask pattern onto the wafer under specified magnifications (reduction rate) with high accuracy. A recent demand on smaller semiconductor devices has accordingly required the image-forming performance to transfer a pattern more minute than was expected in a conventional projection optical system.
Improvements of the resolution for conventional projection exposure apparatuses include use of an increased NA in a projection optical system with a fixed wavelength of exposure light, and a shortened exposure wavelength, for example, from the g line through the i line to an excimer laser beam (with a wavelength of 300 nm or less). More recent approaches use of a phase shift mask and a modified illumination to mitigate limits on the exposure resolution.
A shorter exposure wavelength narrows or limits a scope of materials usable for the projection optical system in view of cost and performance, such as transmittance, durability, etc.
Thus, projection lenses used for exposure apparatuses in the generation after, for example, the ArF excimer laser (with a central wavelength of 193 nm) need to have a simple configuration which decreases the number of transmitting members. A projection optical system with a simple configuration would decrease the number of parameters at the time of lens adjustment, and accordingly would have a less freedom of adjustment. Therefore, it would become difficult to attain a high performance goal. On the other hand, as the miniaturization advances, projection lenses with a higher performance than ever are needed, and the difficulty of the projection lens fabrication for the shortened exposure wavelength is drastically increasing.
In the meanwhile, a mask used in the projection exposure apparatus is provided with a pellicle film (or particle-proof film), i.e., a dustproof transparent member so that foreign particles such as dirt and dust may not adhere to a circuit pattern surface. This helps to improve a yield of products.
The pellicle film is one of those members which have the above problems with the shortened exposure wavelength. One proposed solution for this problem is to use a plane parallel plate of glass material with a given thickness, not a film, using the conventional materials.
However, a transparent member of such a plane parallel plate would flex because of its own weight when applied to an exposure apparatus, and a pattern (a circuit pattern image) projected by a projection optical system would include various aberrations, especially, a distortion, due to a transformation by transparent member's own weight. Typically, since such a dustproof transparent member is fixed to a mask via a rectangular frame, a flexure distribution in a plane of this transparent member will be as shown in FIG. 3. FIG. 3 shows numeric representation of a flexure in each area. FIG. 3 shows the shape of a pellicle associated with an exposure area of a ¼ fold reduction scanner (that is a scanning projection exposure apparatus having a magnification of the projection optical system of ¼), its size being 104 mm×132 mm with a thickness of 200 μm, and a vertical interval between its top and bottom being approximately 34 nm. A distortion caused by these flexures is as shown in FIG. 4, and even if a correction using an approach proposed in Japanese Laid-Open Patent Application No. 4-130711, a rotationally symmetrical error component could be corrected, but a certain amount of components would remain uncorrected as shown in FIG. 5.
The distortion correction mechanism proposed in the above reference is one mechanism in a projection exposure apparatus, which changes the pressure in the air chamber in the projection lens, and corrects a change in projection optical system's magnification based on the transformation (flexure) of the dustproof transparent member and that a wafer surface follows a post-fluctuation focal point in relation to the fluctuation in the focal point of the projection optical system based on the transformation.
On the other hand, if a circuit pattern written onto a mask plate is deviated from a design value of the circuit (or an ideal circuit pattern), it would result in a transfer of a distorted circuit onto the wafer. Accordingly, the projection exposure apparatus including a means for correcting deviances (writing errors) of the circuit pattern written on the mask from the design values is proposed in Japanese Laid-Open Patent Application No. 7-29803.